The present invention relates to a coating method for the preparation of coated nuclear fuel. Particularly, the present invention relates to the coating method of nuclear fuel surface with more than two coated layers of carbides, borides or nitrides, comprising steps of depositing or permeating i) elements or compounds that can form carbides, borides or nitrides by reacting with pyrolytic carbon, boron or nitrogen and ii) a layer of pyrolytic carbon or boron prepared by chemical vapor deposition(CVD) or sputtering in sequence or in reverse sequence, or nitrogen prepared by gas permeation in sequence, on the nuclear fuel surface. The coated layers are formed with carbides, borides, nitrides or their mixtures at high temperature and pressure by combustion synthesis.
Various types of nuclear power plants have been developed in different concepts such as High Temperature Gas-cooled Reactor (HTGR), Pressurized Light Water Reactor (PLWR) and Pressurized Heavy Water Reactor (PHWR). Nuclear fuels used in the nuclear power plants depend on such reactor types. For example, spherical shape nuclear fuels are used for High Temperature Gas-cooled Reactor (HTGR) and cylindrical pellet type nuclear fuels are normally used in Water-cooled Reactors which are the most popular nuclear power plants in operation worldwide.
In order to secure the safety of high temperature gas-cooled reactor, deposition of graphite and carbide on the nuclear fuel kernel was applied to avoid fission gas release. The coated layers were prepared on the surface of spherical nuclear fuel kernel with a thickness of hundreds micron(.mu.m). For this preparation, the coating on spherical nuclear fuels(UCO, ThO.sub.2) was carried out with a mixture of methylchlorosilane and ethylene gases in a fluidized bed type chamber by a chemical vapor deposition method (FBT CVD): pyrolytic carbon and silicon carbide layers are formed at high temperatures of about 1200.degree. C. and 1600.degree. C., respectively. The problems of these methods are the occurrence of microstructural defects due to relatively high temperature employed and the formation of hazardous by-products such as hydrochloric acid and chlorine gas due to the thermal decomposition of silane compounds such as methylchlorosilane, dimethylchlorosilane and triethylchlorosilane.
Meanwhile, in water-cooled reactors, used is relatively large-sized pellet type nuclear fuel in cylindrical shape, whose dimension varies from 7 to 12 mm in diameter and from 10 to 15 mm in height with dish-shaped grooves in its upper and lower faces. Coating of carbides, borides or nitrides on this pellet type nuclear fuel can be thought to effectively reduce the fission gas release, particularly in high-burnup fuel. Coated pellet type nuclear fuel has never been used and the coating method for this type fuel has not been yet envisaged. Furthermore, conventional FBT CVD method can not be applied to pellet type nuclear fuel which has anisotropic shape and is relatively larger and heavier than spherical type nuclear fuel.
The present inventors have found a coating method for nuclear fuel surface with more than two coated layers of carbides, borides, nitrides or their mixture comprising deposition or permeation steps of i) elements or compounds that can form carbides, borides or nitrides by reacting with pyrolytic carbon, boron or nitrogen and ii) a layer of pyrolytic carbon or boron prepared by chemical vapor deposition(CVD) or sputtering in sequence or in reverse sequence, or nitrogen prepared by gas permeation in sequence, on the nuclear fuel surface. The coated layers are formed with carbides, borides, nitrides or their mixture at high temperature and pressure by combustion synthesis.